1. Field of the Invention
In general, the present invention relates to a method of fabrication of semiconductor processing apparatus from an aluminum substrate. In particular, the invention relates to a method of forming complex shapes which can subsequently be anodized to provide at least one plasma-resistant surface, and particularly a surface which is resistant to halogen-containing plasmas.
2. Brief Description of the Background Art
Semiconductor processing involves a number of different chemical and physical processes whereby minute integrated circuits are created on a substrate. Layers of materials which make up the integrated circuit are created by chemical vapor deposition, physical vapor deposition and epitaxial growth, for example. Some of the layers of material are patterned using photoresist masks and wet and dry etching techniques. Patterns are created within layers by the implantation of dopants at particular locations. The substrate upon which the integrated circuit is created may be silicon, gallium arsenide, glass, or any other appropriate material.
Many of the semiconductor processes used to produce integrated circuits employ halogen or halogen-containing gases or plasmas which are particularly corrosive to processing apparatus surfaces they contact. Some processes use halogen-containing liquids. In addition, since the processes used to create the integrated circuits leave contaminant deposits on the surfaces of the processing apparatus, such deposits are commonly removed using plasma cleaning techniques which employ at least one halogen-containing gas, which is also corrosive.
Aluminum has been widely used as a construction material for semiconductor fabrication equipment, at times because of its conductive properties, and generally because of ease in fabrication and availability at a reasonable price. However, aluminum is susceptible to reaction with halogens such as chlorine, fluorine, and bromine, to produce, for example, AlCl3 (or Al2Cl6); or AlF3; or AlBr3 (or Al2Br6). The aluminum-fluorine compounds can flake off the surfaces of process apparatus parts, causing an eroding away of the parts themselves, and serving as a source of particulate contamination of the process chamber (and parts produced in the chamber). Most of the compounds containing aluminum and chlorine and many of the compounds containing aluminum and bromine are gaseous under semiconductor processing conditions and leave the aluminum structure, creating voids which render the structure unstable and with a surface having questionable integrity.
Porosity of the surface of aluminum semiconductor processing apparatus is of grave concern. We discovered that when a large block of aluminum alloy such as 6061 is machined out (hogged out) to produce a complex shape Such as a liner for a processing vessel, the machined surface exhibits a high degree of porosity. We considered extruding a tube, as a technique for obtaining an improved aluminum grain structure, and then joining the tube to a top plate to obtain a process chamber liner shape. However, then the problem shifts to joining of the tube to the top plate. Welding of the tube to the top plate typically creates impurities at the interface of materials coming together in the weld (at the joint of the weld), and the impurities frequently increase porosity at the weld joint. The impurities may be in the form of a filler material used in the welding process or may be in the form of impurities present in the aluminum alloy itself which migrate to the weld joint area during the welding process. Welding is generally defined as a coalescence of metals produced by heating to a suitable temperature with or without the application of pressure, and with or without the use of a filler material. Some of the more commonly used welding techniques include electron-beam welding, laser welding, and solid-phase welding. Solid phase welding processes include, for example, diffusion bonding, friction welding, and ultrasonic joining. Solid phase processes typically produce welds without melting the base material and without the addition of a filler material. Pressure is always employed, and generally some heat is provided. Furnace heating is generally provided in diffusion bonding while frictional heat is developed in ultrasonic and friction joining.
Welding typically produces stress both at the welding joint and in material adjacent the welding joint. Heat treatment or annealing is commonly used to relieve stress. Aluminum alloys begin to exhibit grain growth at temperatures approaching 345° C., which causes precipitation of non-aluminum metals at the grain boundaries. This precipitation may lead to cracking along a weld joint when the weld joint is mechanically loaded, and may lead to cracking along grain boundaries during machining. The precipitation also reduces mechanical properties of the alloy by affecting the uniformity of the alloy composition within the article.
If an aluminum alloy is to perform well in a number of semiconductor process apparatus applications, it should also have desirable mechanical properties. Further, mechanical properties should enable machining to provide an article having the desired final dimensions. For example, if the alloy is too soft, it is difficult to drill a hole, as material tends to stick during the drilling rather than to be removed by the drill. Controlling the dimensions of the machined article is more difficult. There is a penalty in machining cost. In addition, the mechanical properties of the article affect the ability of the article to perform under vacuum, depending on the function of the article. For example, a process chamber must exhibit sufficient structural rigidity and resistance to deformation that it can be properly sealed against high vacuum.
With respect to resistance to a halogen-containing plasma, a preferred means of protection of the aluminum surfaces within a process apparatus has been an anodized aluminum coating. Anodizing is typically an electrolytic oxidation process that produces an integral coating of relatively porous aluminum oxide on an aluminum surface. Despite the use of anodized aluminum protective layers, the lifetime of anodized aluminum parts in semiconductor processing apparatus has been limited, due to the gradual degradation of the protective anodized film. In addition, in the past, the combination of mechanical performance of the article and corrosion resistance of the surface of the article has not been adequately addressed. In attempting to obtain the mechanical properties required for the aluminum alloy body of an article, it is possible to affect the surface of the aluminum alloy in a manner such that the aluminum oxide (anodized) layer does not form a proper interface with the aluminum alloy. This creates a porosity, for example gaps between the aluminum oxide layer and the underlying aluminum surface. We have determined that it is particularly difficult to form a protective anodized coating over a weld joint. The porosity and impurities present at a conventional weld joint interfere with anodization of the aluminum present at the weld joint surface. This porosity promotes a breakdown in the protective aluminum oxide layer, leads to particle formation and a constantly accelerating degradation of the protective aluminum oxide film.
Not only is there significant expense in equipment maintenance and apparatus replacement costs due to degradation of the protective aluminum oxide film, but if a susceptor, for example, develops significant surface defects, these defects can translate through a silicon wafer atop the susceptor, creating device current leakage or even short. The loss of all the devices on a wafer can be at a cost as high as $50,000 to $60,000 or more.
It is readily apparent from the above discussion that there is a long standing need for a method of producing semiconductor apparatus components which have a complex shape (the component is not merely a flat plate, for example), which have adequate mechanical properties for the intended application, and which are protected by an anodized coating which is capable of withstanding a corrosive plasma environment.